In recent years, as a transmission for a vehicle, a belt type continuously variable transmission, in which a drive belt is wound around a primary pulley and a secondary pulley and power is transmitted using a frictional force generated between each pulley and the drive belt by thrust applied to a movable sheave of each pulley, has been put to practical use.
In a case of the continuously variable transmission, it is required to secure the frictional force by increasing the thrust when transmitting great power. At this time, load of a driving source (an engine or an electric motor) for driving a thrust-generating oil pump is increased, then this leads to increase in a fuel consumption amount or a power consumption amount. Further, durability of each pulley and the drive belt etc. might be deteriorated.
Therefore, there has been developed a continuously variable transmission mechanism that transmits power by a plurality of pinion sprockets and a chain wound around the pinion sprockets without using the thrust and the frictional force.
Such a continuously variable transmission mechanism is provided, at an input side and an output side thereof, with an apparent large sprocket (here, called a composite sprocket) formed by the plurality of pinion sprockets, each of which is supported movably in a radial direction with equidistance of each pinion sprocket from a rotation shaft maintained and each of which revolves about the rotation shaft so as to rotate integrally with each other and also each of which is arranged at an apex of a polygon. Then, by the chain wound around these composite sprockets, the power is transmitted. In such a configuration, the pinion sprockets radially move in synchronization with each other while maintaining their equidistance from the rotation shaft, thereby changing a size of the polygon as a similar shape, then a transmission ratio is changed.
At a time of the transmission ratio change (or speed change) in such continuously variable transmission mechanism, excess and deficiency of a chain length occur between the pinion sprockets due to the fact that a distance between the pinion sprockets changes, and thus slack and tension of the chain occur. Unless these excess and deficiency of the chain length are overcome, it is not possible to radially move each pinion sprocket and to change the transmission ratio.
For this problem, there has been developed technique that is disclosed in the following Patent Document 1 and Patent Document 2.
Patent Document 1 discloses that two disks (two spindles) are arranged parallel to each other at one side of a plurality of pinion sprockets, radial slots are formed on each disk, the radial slot (hereinafter, called a first radial slot) on one disk and the radial slot (hereinafter, called a second radial slot) on the other disk are arranged so as to intersect with each other, and a shaft of each sprocket is supported in a position where the first and second radial slots intersect. When a relative angle (a relative phase) between one disk and the other disk is changed, since an intersection position of the first and second radial slots moves in a radial direction, the shaft of each pinion sprocket which is supported in this intersection position moves in the radial direction by and according to the relative rotation of the both disks.
Patent Document 1 also discloses that the pinion sprocket has a predetermined mechanism in which when a force by torque transmission through a chain acts on the pinion sprocket, the chain is engaged with the pinion sprocket, then the pinion sprocket is allowed to rotate integrally with an axis of the continuously variable transmission mechanism, and in which when the force by the torque transmission through the chain does not act on the pinion sprocket and the force is released, the chain is disengaged from the pinion sprocket, then the pinion sprocket is allowed to rotate freely with respect to the axis of the continuously variable transmission mechanism. When the predetermined mechanism is in an engagement state, the power transmission becomes possible, and when the predetermined mechanism is in a disengagement state, a radial direction movement of each pinion sprocket becomes possible. Further, a chain tensioner giving a tension to the chain to operate the predetermined mechanism is provided.
Patent Document 2 discloses a power distribution mechanism in which a female thread is provided at a slide frame to which a pinion sprocket is fixed and a male screw mating with this female thread is rotated. By simultaneously rotating each of the same number of male screw by the power distribution mechanism, each sprocket is moved in a radial direction.
Patent Document 2 also discloses that each pinion sprocket is provided with a reverse rotation preventing unit that permits rotation in one direction and prohibits rotation in the other direction and a clutch that switches between fixation and release of the rotation. By the reverse rotation preventing unit, it is possible to adjust the excess and deficiency of the chain length when changing a position of each pinion sprocket in the radial direction. Further, by fixing the rotation of each pinion sprocket by the clutch, engine brake becomes effective.
According to the predetermined mechanism of Patent Document 1, since the power transmission and the transmission ratio change (the speed change) are performed exclusively, it is difficult to change the transmission ratio at a time of the power transmission, and it is difficult to transmit the power at a time of the change of the transmission ratio. In addition, as a problem, even at a constant transmission ratio, the engine brake is ineffective. Moreover, since the chain tensioner is provided, a power transmission efficiency might be decreased due to friction generated between the chain and the chain tensioner.
On the other hand, according to the reverse rotation preventing unit of Patent Document 2, when the rotation of each pinion sprocket is fixed by the clutch, it is impossible to change the transmission ratio. Further, when the rotation of each pinion sprocket is not fixed by the clutch, although the transmission ratio can be changed, there is a possibility that the power cannot be transmitted because of free rotation of each pinion sprocket. Furthermore, as a problem, the engine brake becomes ineffective.